title: Roles of BOR2, a boron exporter, in cross linking of rhamnogalacturonan II and root elongation under boron limitation in Arabidopsis.
abstract: Boron (B) is required for cross linking of the pectic polysaccharide rhamnogalacturonan II (RG-II) and is consequently essential for the maintenance of cell wall structure. Arabidopsis (Arabidopsis thaliana) BOR1 is an efflux B transporter for xylem loading of B. Here, we describe the roles of BOR2, the most similar paralog of BOR1. BOR2 encodes an efflux B transporter localized in plasma membrane and is strongly expressed in lateral root caps and epidermis of elongation zones of roots. Transfer DNA insertion of BOR2 reduced root elongation by 68%, whereas the mutation in BOR1 reduced it by 32% under low B availability (0.1 µm), but the reduction in shoot growth was not as obvious as that in the BOR1 mutant. A double mutant of BOR1 and BOR2 exhibited much more severe growth defects in both roots and shoots under B-limited conditions than the corresponding single mutants. All single and double mutants grew normally under B-sufficient conditions. These results suggest that both BOR1 and BOR2 are required under B limitation and that their roles are, at least in part, different. The total B concentrations in roots of BOR2 mutants were not significantly different from those in wild-type plants, but the proportion of cross-linked RG-II was reduced under low B availability. Such a reduction in RG-II cross linking was not evident in roots of the BOR1 mutant. Thus, we propose that under B-limited conditions, transport of boric acid/borate by BOR2 from symplast to apoplast is required for effective cross linking of RG-II in cell wall and root cell elongation.
authors.affiliation: Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan.
authors.forename: Kyoko
authors.initials: K
authors.lastname: Miwa
authors.forename: Shinji
authors.initials: S
authors.lastname: Wakuta
authors.forename: Shigeki
authors.initials: S
authors.lastname: Takada
authors.forename: Koji
authors.initials: K
authors.lastname: Ide
authors.forename: Junpei
authors.initials: J
authors.lastname: Takano
authors.forename: Satoshi
authors.initials: S
authors.lastname: Naito
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Omori
authors.forename: Toshiro
authors.initials: T
authors.lastname: Matsunaga
authors.forename: Toru
authors.initials: T
authors.lastname: Fujiwara
mesh_terms: D027321:Anion Transport Proteins / Q000378:metabolism*; D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D001895:Boron / Q000494:pharmacology*; D048429:Cell Size / Q000187:drug effects; D004269:DNA, Bacterial / Q000235:genetics; D019281:Dimerization; D008954:Models, Biological; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional / Q000235:genetics; D009154:Mutation / Q000235:genetics; D009928:Organ Specificity / Q000187:drug effects; D010368:Pectins / Q000378:metabolism*; D019441:Plant Epidermis / Q000166:cytology / Q000187:drug effects; D018517:Plant Roots / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D030821:Plants, Genetically Modified; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000378:metabolism; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism
chemical_list: D027321:Anion Transport Proteins; D029681:Arabidopsis Proteins; C000590493:BOR2 protein, Arabidopsis; D004269:DNA, Bacterial; C036483:T-DNA; C042492:rhamnogalacturonan II; D010368:Pectins; D001895:Boron